Copolymers containing repeating units derived from isoolefins are industrially prepared by carbocationic polymerization processes. Of particular importance is butyl rubber which is a copolymer of isobutylene and a smaller amount of a multiolefin such as isoprene.
The carbocationic polymerization of isoolefins and its copolymerization with multiolefins is mechanistically complex. The catalyst system is typically composed of two components: an initiator and a Lewis acid such as aluminum trichloride which is frequently employed in large scale commercial processes.
Examples of initiators include proton sources such as hydrogen halides, carboxylic acids and water.
During the initiation step, the isoolefin reacts with the Lewis acid and the initiator to produce a carbenium ion which further reacts with a monomer forming a new carbenium ion in the so-called propagation step.
The type of monomers, the type of diluent or solvent and its polarity, the polymerization temperature as well as the specific combination of Lewis acid and initiator affects the chemistry of propagation and thus monomer incorporation into the growing polymer chain.
Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber, polyisobutylene, etc. in methyl chloride as diluent. Typically, the polymerization process is carried out at low temperatures, generally lower than −90 degrees centigrade. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of up to 35 wt.-% in the reaction mixture can be achieved, as opposed to a polymer concentration of typically at maximum 20 wt.-% in solution polymerizations. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
The fact that the use of methyl chloride however restricts the range of catalysts that may be employed and further limits the temperature range for the polymerization to obtain the desired high molecular weights leads to products with low variability of the microstructure in particular the multiolefin distribution within the polymer chains. Further, slurry polymerizations in particular in methyl chloride suffer from particle agglogeration and fouling which leads to insufficient removal of the exothermic heat of polymerization and thus to inhomogeneous reaction conditions within the reactor.
Therefore, finding alternative polymerization conditions including specific combinations of initiators and diluents would not only reduce particle agglomeration and reactor fouling but also creation of novel polymers with specific sequence distributions which is highly desirable in industry due to the fact that such polymers would increase the options to influence curing behaviour by changing the distribution of crosslinking sites, in particular in applications where at least two types of rubber are cured simultaneously (co-curing). Moreover such polymers would open up a versatile platform to also produce novel polymers by postpolymerization modification.
As already mentioned above the sequence distribution of the final copolymer is influenced by the polymerization conditions which determine the relative reactivity of the comonomers employed. The sequence distribution of a copolymer may be expressed in terms of combinations of adjacent structural units. For example, characterizable sequences of two monomer units are called diads. Three monomer unit sequences are called triads. Four monomer unit sequences are called tetrads and so forth. Copolymers prepared under different conditions with the same comonomer incorporation may exhibit differences in their sequence distributions as expressed by the diad (or triad, etc.) fractions in the copolymer chain. Sequence distributions and comonomer incorporation are mathematically linked by probability statistics because of the competitive nature of the chemical events involved in copolymerization. A parameter that aids in the characterization of this relationship is the reactivity ratio, a ratio of the rate constants of homopropagation (adding a like monomer) to cross propagation (adding an unlike monomer). Copolymers with the same comonomer incorporation, but with different sequence distributions often exhibit different physical properties. See e.g. Chemical Microstructure of Polymer Chains, J. L. König, Wiley-Interscience, New York, 1980, and Polymer Sequence Determination: Carbon-13 NMR Method, J. C. Randall, Academic Press, 1977. An extreme, but well-known example is the comparison of the physical attributes of random and block copolymers.
It is generally known that conjugated dienes are less reactive than isobutylene in carbocationic copolymerization systems. Of the known linear conjugated dienes, isoprene is one of the more reactive dienes in copolymerization with isobutylene. This tendency towards lower reactivity of the conjugated diene is expressed in the sequence distribution of the prepared copolymers. At a given copolymer composition, isoprene units do not exhibit a tendency to follow other isoprene units in the copolymer chain. Consequently, BII (B=isobutylene, I=isoprene), IIB and III triad fractions are relatively low than compared to systems with more reactive comonomers.
Because isobutylene/isoprene copolymerations are often conducted in chlorinated hydrocarbons or mixtures of hydrocarbons and chlorinated hydrocarbons, the degree to which the sequence distribution can be varied is quite limited. Expression of this limitation is found by examination of the known reactivity ratios of isoprene for isobutylene/isoprene copolymerizations See e.g., J. E. Puskas, “Carbocationic Polymerizations” in Encyclopedia of Polymer Science and Technology, John Wiley and Sons, New York, 2003. Values for isoprene reactivity ratios under a variety of polymerization conditions fall below 1.4 indicating a narrow range of available isoprene centered triad fractions (BII, IIB and III) in the prepared copolymers.
EP 1572 766 A discloses a process to modify the sequence distribution of butyl rubbers by applying fluorinated hydrocarbons as diluent. In particular, EP 1572 766 A discloses copolymers wherein the sequence distribution parameter m, which can be calculated according to equation (I)F=mA/(1+mA)2  (eq. I)wherein                A is the molar ratio of multiolefin to isoolefin in the copolymer as determined by 1H NMR; and        F is the isoolefin-multiolefin-multiolefin triad fraction in the copolymer as determined by 13C NMR; andis either from 1.10 to 1.25 or above 1.5. Specifically as can be seen in a written declaration of Dr. T. D. Shaffer submitted on Feb. 12, 2007 to the file of EP 1572 766 A, the parameter m also heavily depends on the isoprene content incorporated into the copolymer. For example an m-value of as low as 1.1 can only be obtained by incorporation of 15.5 mol-% of isoprene using 1,1,1,2-tetrafluoroethane as a diluent, while polymers with lower contents of e.g. 3.32 mol-% of isoprene obtained in the same diluent exhibit an m-value of 1.3.        
The same document shows that the typical slurry polymerization process performed in methyl chloride leads to copolymers having an m-value of 1.3 (at an isoprene content of 12.7 mol.-%) to 2.1 (at an isoprene content of 2.55 mol.-%).
In view of that, and in order to broaden the variety of different butyl rubbers available for manufacturers of rubber products there is still a need to provide copolymers of isobutylene and multiolefins, in particular isoprene having more uniformly distributed crosslinking sites i.e. an even lower m-value than known from the state of the art.